Refrigeration circuit

ABSTRACT

A refrigeration circuit has a mono- or multi-component refrigerant, especially CO 2 , circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line having a medium pressure holding valve arranged therein is provided connecting the line between the condenser/gascooler and the high pressure control valve to the line between the collecting container and the expansion device(s), and wherein a control unit is provided said control unit being configured to open the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

The invention relates to a refrigeration circuit and to a method of transcritical operation of a refrigeration circuit.

Current CO₂ refrigeration circuits employ a two-stage expansion where the refrigerant is relieved from a high pressure level to a medium pressure level by a first stage expansion device, and where the expansion devices upstream of the evaporators further expand the refrigerant to a suction pressure level.

When operated at ambient temperatures of less than 10° C. such refrigeration circuits often face the problem of maintaining the medium pressure at a sufficiently high level in order that the evaporators downstream can provide adequate cooling performance. Sometimes the medium pressure collapses, and no refrigerant flow gets to the evaporators in the supermarket which can cause substantial damage to the goods to be sold, since they are no more kept in the necessary temperature range.

Accordingly, it would be beneficial to provide a refrigeration circuit and a method for transcritical operation of a refrigeration circuit that enable a reliable and sufficient cooling performance even with low ambient temperatures.

Exemplary embodiments of the invention include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line having a medium pressure holding valve arranged therein is provided connecting the line between the condenser/gascooler and the high pressure control valve to the line between the collecting container and the expansion device(s); and wherein a control unit is provided said control unit being configured to open the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line is provided connecting the line after the high pressure control valve to the line between the collecting container and the expansion device(s); wherein a medium pressure holding valve is arranged in the line after the branch-off point of the bypass line and before the collecting container; and wherein a control unit is provided said control unit being configured to close the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the refrigerant bypasses the collecting container and the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line having a medium pressure holding valve arranged therein is provided connecting the line after the high pressure control valve to the line between the collecting container and the expansion device(s); and wherein a control unit is provided said control unit being configured to open the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that that the refrigerant bypasses the collecting container and the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line is provided connecting the line after the high pressure control valve to the line between the collecting container and the expansion device(s), wherein a medium pressure holding valve is arranged in the line after the collecting container and before the refeed point of the bypass line, and wherein a control unit is provided said control unit being configured to close the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the refrigerant bypasses the collecting container and the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a Refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line having a medium pressure holding valve arranged therein is provided connecting the pressure line after the compressor unit to the line between the condenser/gascooler and the high pressure control valve, and wherein a control unit is provided said control unit being configured to open the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a bypass line having a medium pressure holding valve arranged therein is provided connecting the pressure line to the collecting container or to the flashgas line before the medium pressure control valve, and wherein a control unit is provided said control unit being configured to open the medium pressure holding valve when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, wherein a collecting container heating unit arranged in the collecting container, and wherein a control unit is provided said control unit being configured to effect heating operation of the collecting container heating unit when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow, a compressor unit, a condenser/gascooler with at least one fan leading air over its surface, a high pressure control valve, a collecting container, and at least one evaporator having an expansion device connected upstream thereof, wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, wherein a temperature, pressure or liquid level sensor is provided in or at the collecting container, and wherein a control unit is provided said control unit being configured to lower the performance of the fan(s) when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

Exemplary embodiments of the invention further include a method for transcritical operation of a refrigeration circuit, in which the medium pressure holding valve is switched accordingly when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.

Embodiments of the invention are described in greater detail below with reference to the figures, wherein

FIG. 1 shows a schematic view of a first refrigeration circuit according to an embodiment of the invention;

FIG. 2 shows a schematic view of a second refrigeration circuit according to an embodiment of the invention;

FIG. 3 shows a schematic view of a third refrigeration circuit according to an embodiment of the invention;

FIG. 4 shows a schematic view of a fourth refrigeration circuit according to an embodiment of the invention;

FIG. 5 shows a schematic view of a fifth refrigeration circuit according to an embodiment of the invention;

FIG. 6 shows a schematic view of a sixth refrigeration circuit according to an embodiment of the invention;

FIG. 7 shows a schematic view of a seventh refrigeration circuit according to an embodiment of the invention; and

FIG. 8 shows a schematic view of an eighth refrigeration circuit according to an embodiment of the invention.

FIG. 1 shows a schematic view of a first refrigeration circuit 2.

The first refrigeration circuit 2 and all other refrigeration circuits explained with respect to FIGS. 2-8 below, comprise the following corresponding elements that are designated with like reference numerals. The refrigeration circuits according to the embodiments of the invention as explained below comprise, in the direction of refrigerant flow, a compressor unit 4 having three compressors connected in parallel, a condenser/gas cooler 6 having two fans 8 and respective fan controls 10 for flowing ambient air over its surface, a high pressure control valve 12, a collecting container/receiver 14, where gaseous and liquid refrigerant are separated from each other and collected, and two evaporators 18 and 20 having a respective expansion valve 16, 20 connected upstream thereof.

The condenser/gas cooler 6 acts as a condenser when the refrigerant circuit is operating in a subcritical mode and as a gas cooler when the refrigerant circuit is operating in a transcritical mode. When the ambient temperatures are lower than 10° Celsius, the gas cooler 6 normally works as a condenser/liquefier.

The compressors of the compressor unit 4 can also be connected in series.

A flashgas line 24 having a medium pressure control valve 26 arranged therein connects the gas space, especially a point in the upper portion of the collecting container 14 with the suction line of the compressor unit 4. Further, a sensor 28 is provided in or at the collecting container 14, especially in an upper portion thereof.

The sensor 28 can be configured as temperature sensor measuring the temperature within the collecting container 14, as pressure sensor sensing the pressure within the collecting container 14, as liquid level sensor sensing the liquid level of liquid refrigerant in the collecting container 14 or as a combination thereof.

It is further to be mentioned that the high pressure control valve 12 effects the phase change of the refrigerant and only a small temperature decrease in the order of 1° Celsius.

As a matter of course the number of compressors of the compressor unit 4, the number of the fans 8 and fan controls 10 and the number of evaporators 18 and 22 is only exemplary, and different numbers of such elements can also be provided.

All refrigeration circuits depicted in FIGS. 1-8 and explained with respect to these figures further comprise a control unit that effects the particular medium pressure control operation in case the sensor senses a too low pressure or temperature within the collecting container 14 or a too high level of liquid refrigerant within the collecting container 14. This situation particularly occurs, when the refrigerant flowing from the high pressure control valve 12 into the collecting container 14 is colder than the refrigerant already collected within the collecting container 14. In this case a pressure and temperature decrease within the collecting container 14 can be avoided and an efficient medium pressure control can be provided that guarantees sufficient cooling performance of the evaporators 18 and 22. In a situation when no flash gas is generated and only liquid refrigerant gets into the collecting container 14, the pressure will collapse and the refrigerant circuit will stop operating. This problem is reliably avoided by the refrigeration circuits and the methods for transcritical operation of a refrigerant circuit, according to exemplary embodiments of the invention, as explained below.

Normal pressure levels of the refrigeration circuits depicted in FIGS. 1-8 are as follows: High pressure level between the compressor unit 4 and the high pressure control valve 12: 100-125 bar approx., medium pressure level between the high pressure control valve 12 and the expansion valves 16 and 20: 35-36 bar, and suction pressure level between the evaporators 18 and 22 and the compressor unit 4: 10-26 bar.

Reverting now to the first refrigeration circuit 2, this first refrigeration circuit 2 further comprises a bypass line 30 having a medium pressure holding valve 32 arranged therein. This bypass line 30 connects the line between the condenser/gas cooler 6 and the high pressure control valve 12 to the line portion downstream the collecting container 14 at a position between the collecting container 14 and the expansion valves 16 and 20. In other words, the medium pressure holding valve 32, which can be a solenoid valve, is connected in parallel with the high pressure control valve 12 and the collecting container 14.

If the sensor 28 senses a too low pressure or temperature value within the collecting container 14 or a too high liquid level of the liquid refrigerant within the collecting container 14 it opens the medium pressure holding valve 32 such that liquid refrigerant bypasses the high pressure control valve 12 and the collecting container 14 and flows directly into the line portion downstream of the collecting container 14. Thereby the medium pressure can reliably controlled and it is avoided that refrigerant being colder than the refrigerant collected in the collecting container 14 flows into the collecting container 14.

FIG. 2 shows a schematic view of a second refrigeration circuit 34.

The second refrigeration circuit 34 comprises, instead of the bypass line 30 and the medium pressure holding valve 32, a bypass line 38 connecting the line after the high pressure control valve 12 to the line portion downstream the collecting container 14 at a position between the collecting container 14 and the expansion valves 16 and 20, and a medium pressure holding valve 36 being arranged in the line after the branch off point of the bypass line 38 and before the collecting container 14. In other words, a bypass line 38 bypasses the collecting container 14 and the medium pressure holding valve 36 is put in series after the high pressure control valve 12 and after the branch off point of the bypass line 38.

If the temperature or pressure in the collecting container 14 sensed by the sensor 28 falls below a pre-determined threshold or the liquid level in the collecting container sensed by the sensor 28 exceeds a predetermined threshold, the control unit closes the medium pressure holding valve 36 such that the refrigerant bypasses the collecting container 14 over the bypass line 38. In this way a temperature or pressure decrease in the collecting container 14 can be avoided and the medium pressure within the collecting container can be kept constant.

In particular the bypass pipe needs to be located above the collecting container 14.

FIG. 3 shows a schematic view of a third refrigeration circuit 40.

Compared to the second refrigeration circuit 34, the third refrigeration circuit 40 lacks the medium pressure holding valve 36 and has a medium pressure holding valve 42 arranged in the bypass line 38 instead. In other words, the medium pressure holding valve 42 is incorporated to bypass the collecting container 42.

When the temperature or pressure in the collecting container 14 gets too low or the liquid level gets too high, the control unit opens the medium pressure holding valve 42 and refrigerant bypasses the collecting container 14. By such operation a temperature or pressure decrease in the collecting container 14 can be reliably avoided. Furthermore a gas phase within the collecting container 14 can be maintained.

In particular the bypass line 38 is placed below the collecting container 14.

FIG. 4 shows a schematic view of a fourth refrigeration circuit 44.

Compared to the second refrigeration circuit 34, the medium pressure holding valve 36 upstream of the collecting container 14 is omitted, and a medium pressure holding valve 46 is provided in the line portion downstream of the collecting container 14 at a position after the collecting container 14 and before the refeed point of the bypass line 38.

When the temperature or pressure in the collecting container 14 gets too low or the liquid level gets too high, the control unit closes the medium pressure holding valve, and liquid refrigerant bypasses the receiver 14 via the bypass line 38.

To ensure proper functionality during bypass and normal operation, a phase change separator can be placed at the branch off point of the bypass line 38 before the collecting container 14.

FIG. 5 shows a schematic view of a fifth refrigeration circuit 48.

Instead of bypassing the collecting container 14 as realized by the refrigeration circuits 2, 34, 40 and 44 depicted in FIGS. 1-4, the fifth refrigeration circuit 48 provides a bypass line 50 in parallel to the condenser/gas cooler 6. In such bypass line 50, a medium pressure holding valve 52 is arranged.

When the temperature or pressure in the collecting container 14 gets too low or the liquid level gets too high, the control unit opens the medium pressure holding valve 52 such that hot pressurized refrigerant bypasses the condenser/gas cooler 6 and mixes with the refrigerant having passed the condenser/gas cooler 6 at or after the refeed point of the bypass line 50.

By this embodiment it is ensured that hot gaseous refrigerant reaches the collecting container 14, and therefore the temperature and pressure in the collecting container 14 can be increased or at least kept constant and a sufficient amount of gaseous refrigerant in the collecting container 14 can be provided.

FIG. 6 shows a schematic view of a sixth refrigeration circuit 54.

The sixth refrigeration circuit 54 comprises a bypass line 56 connecting the pressure line after the compressor unit 4 with the flash gas line 24 at a position close to the collecting container 14 and before the medium pressure control valve 26.

When the temperature or pressure within the collecting container 14 gets too low or the liquid level gets too high, the control unit actuates or opens the medium pressure holding valve 58 such that hot gaseous refrigerant flows from the pressure line into the gas space of the collecting container 14. By this embodiment, a temperature or pressure decrease in the collecting container 14 can be avoided, and a sufficient amount of gaseous refrigerant within the collecting container 14 can be provided.

FIG. 7 shows a schematic view of a seventh refrigeration circuit 60.

Instead of providing bypass lines and an additional medium pressure holding valve, the seventh refrigeration circuit 60 provides two medium pressure control embodiments, that can be employed independently from each other.

As first embodiment, the sixth refrigeration circuit 60 comprises a collecting container heating unit 62, which in case the temperature or pressure within the collecting container 14 gets too low or the liquid level gets too high, is switched on by the control unit and heats the refrigerant within the collecting container 14. In particular at high liquid level boiling of the refrigerant will supply sufficient refrigerant gas formation. Hence, a pressure or temperature decrease within the collecting container 14 can be avoided, and a sufficient amount of gaseous refrigerant in the collecting container 14 can be provided.

According to a second, alternative embodiment, the control unit lowers the performance of at least one of the fans 8, when the temperature or pressure in the collecting container 14 gets too low or the liquid level gets too high. By such lowering of the fan performance, the efficiency of the condenser/gas cooler 6 is lowered such that the heat extraction within the condenser/gas cooler is reduced which will supply sufficient refrigerant in gaseous form to the collecting container 14. Such lowered performance of the condenser/gas cooler 6 can either be achieved by switching off fans or stage-wise reduction of the fans running, or a variable speed drive can be provided or integrated in the control unit that continuously lowers the performance of the fans 8.

FIG. 8 shows a schematic view of an eighth refrigeration circuit 64.

The eighth refrigeration circuit 64 comprises, as an exemplary embodiment, all medium pressure control units of the first to seventh refrigeration circuits 2, 34, 40, 44, 48, 54 and 60, namely the collecting container bypass line 30 and the medium pressure holding valve 32, the collecting container bypass line 38 and the medium pressure holding valves 36, 42 and 46, the condenser/gas cooler bypass line 50 and the medium pressure holding valve 52, the bypass line 56 and the medium pressure holding valve 58, the collecting container heating unit 62 and the fan performance control.

In the eighth refrigeration circuit 64 one, two or more or even all of those medium pressure control units can be employed as appropriate.

It is explicitly disclosed therein, that the medium pressure control units described with respect to FIGS. 1-7 can be combined in any fashion, and the right to direct claims on any of such combination of measurement units is explicitly reserved.

In all embodiments of the refrigeration circuit, as described herein, at least one of a pressure sensor, a temperature sensor and a liquid level sensor is provided in or at the collecting container 14, and the sensed values are transmitted to and used by the control unit which in turn actuates the respective medium pressure holding valves 32, 36, 42, 46, 52 and 58, and/or the collecting container heating unit 62 and/or the fan(s) 8.

In general, the refrigerant circuits of the invention, as described herein, can be operated with any conventional refrigerant. Particularly, CO₂ can be circulated within the refrigerant circuits. More particularly, CO₂ can be used as refrigerant, and the refrigeration circuit can be operated in a transcritical mode.

According to exemplary embodiments of the invention, as described herein, a reliable and energy efficient medium pressure control of transcritical refrigeration systems, especially transcritical CO₂ refrigeration systems within the collecting container in the event of low ambient temperatures is attained. All exemplary embodiments of the invention, as described herein, have one more benefit: they save on energy consumption.

By the high pressure control valve 12 which acts as a first stage expansion device and reduces the pressure of the cooled refrigerant before entering the refrigerant collecting container 14 to the medium pressure level high pressure refrigerant can be avoided from entering the supermarket. The liquid refrigerant from the collecting container 14 is sent to the evaporators 18, 22 in the supermarket at the medium pressure level to be used in the individual cooling cabinets. The vapor from the collecting container 14 is expanded to the suction pressure return line by the medium pressure control valve 26.

By the refrigerant circuits according to exemplary embodiments of the invention, as described herein, a reliable transcritical operation at ambient temperatures of less than 10° Celsius can be attained. Problems as occurring in other refrigeration circuits regarding a medium pressure decrease due to lack of vapor at the high pressure control valve 12 can reliable avoided and a sufficient flow of refrigerant at an appropriate pressure to the cabinets in the supermarket is ensured.

Under normal conditions it suffices that the medium pressure control units, as described above, are operated for a comparatively short time frame of a few seconds in order to avoid a temperature or pressure decrease within the collecting container 14 and to ensure that enough gaseous refrigerant is present within the collecting container 14. For this purpose the medium pressure holding valves 32, 36, 42, 46, 52, and 58 can be configured as solenoid valves.

In the claims the features of the refrigeration circuit are defined in detail for the refrigeration circuit. As a matter of course, such features can also be present, as corresponding method steps for the respective methods for transcritical operation of a refrigeration circuit, and for such method claims the same advantages apply as for the respective refrigeration circuit. The right to add such method claims at a later stage of procedure is explicitly reserved.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition many modifications may be made to adopt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention include all embodiments falling within the scope of the dependent claims. 

1. A refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow: a compressor unit; a condenser/gascooler; a high pressure control valve; a collecting container; and at least one evaporator having an expansion device connected upstream thereof; wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and a suction line leading to the compressor unit, characterized in that: a temperature, pressure or liquid level sensor is provided in or at the collecting container; a medium pressure holding valve is provided; and a control unit is provided, said control unit being configured to open the medium pressure holding valve when the temperature or pressure in the collecting, container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.
 2. The refrigeration circuit of claim 1, wherein the medium pressure holding valve is disposed in a bypass line connecting a line between the condenser/gascooler and the high pressure control valve to a line between the collecting container and the expansion device(s).
 3. The refrigerant circuit of claim 1 further comprising a bypass line is provided connecting a line after the high pressure control valve to a line between the collecting container and the expansion device(s).
 4. The refrigerant circuit of claim 3 wherein the medium pressure holding valve is arranged in the line after a branch-off point of the bypass line and before the collecting container.
 5. The refrigeration circuit of claim 3 wherein the medium pressure holding valve is disposed in the bypass line.
 6. The refrigeration circuit of claim 3 wherein the medium pressure holding valve is arranged in a line after the collecting container and before a refeed point of the bypass line.
 7. The refrigeration circuit of claim 6, wherein a phase-change separator is placed at the branch-off point of the bypass line.
 8. The refrigeration circuit of claim 1, further comprising: a bypass line connecting the pressure line after the compressor unit to the line between the condenser/gascooler and the high pressure control valve; wherein the medium pressure holding valve is arranged in the bypass line.
 9. The refrigeration circuit of claim 1, further comprising: a bypass line having connecting a pressure line to the collecting container or to a flashgas line before the medium pressure control valve; wherein the medium pressure holding valve is arranged in the bypass line.
 10. The refrigeration circuit of claim 1, wherein the compressor unit comprises a set of compressors.
 11. The refrigeration circuit of claim 1, wherein at least two evaporators having a respective expansion device connected upstream thereof are connected in parallel.
 12. A refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow: a compressor unit; a condenser/gas cooler; a high pressure control valve; a collecting container; and at least one evaporator having an expansion device connected upstream thereof; wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and a suction line leading to the compressor unit, characterized in that: a temperature, pressure or liquid level sensor is provided in or at the collecting container; a collecting container heating unit arranged in the collecting container; and a control unit is provided said control unit being configured to effect heating operation of the collecting container heating unit when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.
 13. The refrigeration circuit of claim 12, wherein the compressor unit comprises a set of compressors.
 14. The refrigeration circuit of claim 12, wherein at least two evaporators having a respective expansion device connected upstream thereof are connected in parallel.
 15. A refrigeration circuit having a mono- or multi-component refrigerant, especially CO₂, circulating therein, said refrigeration circuit enabling an transcritical operation, said refrigeration circuit comprising, in the direction of refrigerant flow: a compressor unit; a condenser/gascooler with at least one fan leading air over its surface; a high pressure control valve; a collecting container; and at least one evaporator having an expansion device connected upstream thereof; wherein a flashgas line having a medium pressure control valve arranged therein is provided between an upper portion of the collecting container and the suction line leading to the compressor unit, characterized in that: a temperature, pressure or liquid level sensor is provided in or at the collecting container; and a control unit is provided said control unit being configured to lower the performance of the fan(s) when the temperature or pressure in the collecting container sensed by the sensor falls below a predetermined threshold or the liquid level in the collecting container sensed by the sensor exceeds a predetermined threshold, such that the medium pressure is prevented from decreasing.
 16. The refrigeration circuit of claim 15, wherein the control unit comprises a variable speed drive for the fan(s).
 17. The refrigeration circuit of claim 15, wherein the compressor unit comprises a set of compressors.
 18. The refrigeration circuit of claim 15, wherein at least two evaporators having a respective expansion device connected upstream thereof are connected in parallel. 